Talk with Fred and George.

"Hi, Harry", says Fred, a cheery grin on his face. "How are you?"

"Not bad", you admit. "And you two?"

"We're good", George replies for both twins.

"We have a business proposition for you", Fred says, his smile dropping from his face as it turns serious.

"A very important proposition", George continues, his voice down to a whisper.

"What kind of proposition?" you ask, curious in spite of yourself.

You know that nothing the twins propose will be wholly safe, but it doesn't harm to ask. Fred and George lean in.

"You know we are making our Weasleys' Wizard Wheezes", Fred says, "and we wanted to know if you'll test one out for us."

"You mean, like that Canary Cream that Neville ate after I defeated the Horntail?" you ask.

"Yeah", Fred replies.

"But we need to test them", George explains. "We've tested this one on us already..."

"We normally get Lee to help us out", Fred picks up where George left off, "but he's in the hospital wing. He has a blood nose."

"What did he eat?" you ask suspiciously.

"Nothing", George says quickly. "He just... er... ran into a table."

"We bet he couldn't jump over the table in the Gryffindor common room", Fred explains, "and we were right."

"So, we came to you", George says, returning the conversation to their test. "We know you're reliable."

Fred reaches into a pocket in his robe, and takes out two small vials, one blue, one green.